Binary Tree Preorder Traversal [Leetcode]

Given a binary tree, return the preorder traversal of its nodes' values.
For example:
Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

return [1,2,3].
Note: Recursive solution is trivial, could you do it iteratively?

Solution: when it comes to the binary tree traversal, we need a stack if the node in the tree has no parent pointer. The idea of the solution 1 is quite simple and I found it online:

1. Push the root into stack;
2. Do the follwoing steps until the stack is empty:
• Pop out the top-most node in the stack as the current node; and print it;
• Push its right child and then left child into stack if they exist.

Solution 2 is from me. Since it is quite complicated compared with Solution 1, I do not discuss it.

   /*******************************SOLUTION 1***********************************/

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> preorderTraversal(TreeNode *root) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        vector<int> ret;
        if(root==NULL)
            return ret;
        stack<TreeNode*> st;
        st.push(root);
        while(!st.empty()){
            TreeNode* cur = st.top();
            st.pop();
            ret.push_back(cur->val); //or print it on the fly
            if(cur->right!=NULL)
                st.push(cur->right);
            if(cur->left!=NULL)
                st.push(cur->left);
        }
        return ret;
    }
};

   /*******************************SOLUTION 2***********************************/

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> preorderTraversal(TreeNode *root) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        vector<int> ret;
        if(root==NULL)
            return ret;
        stack<TreeNode*> st;
        TreeNode* cur = root;
        do{
            ret.push_back(cur->val);
            if(cur->left==NULL && cur->right==NULL){
                if(!st.empty()){
                    cur=st.top()->right;
                    st.pop();
                }
                else
                    cur=NULL;
            }
            else if(cur->left!=NULL && cur->right!=NULL){
                st.push(cur);
                cur=cur->left;
            }
            else if(cur->left!=NULL)
                cur=cur->left;
            else
                cur=cur->right;
        }while(!st.empty() || cur!=NULL);
        return ret;
    }
};